Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2000-03-22

2004-12-28

Vannucci, James (Department: 2821)

Coherent light generators

Particular active media

Semiconductor

C372S046012, C438S478000, C257S094000

Reexamination Certificate

active

06836496

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device made of a group III-V nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) such as BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride) or TIN (thallium nitride) or mixed crystals thereof.
2. Description of the Prior Art
In recent years, GaN based semiconductor light emitting devices are increasingly put into practice as semiconductor laser devices emitting blue or violet light. Such semiconductor laser devices are generally employed as the light sources for optical disk systems writing and reading information in and from an optical disk among optical memories optically writing or reading information. In particular, GaN based semiconductor laser devices are expected as the light sources for high-density optical disk systems such as advanced digital video disks.
FIG. 12
is a typical sectional view of a conventional GaN based semiconductor laser device. Referring to
FIG. 12
, an undoped GaN low-temperature buffer layer
52
, an undoped GaN layer
53
, an n-GaN layer
54
, an n-anti-cracking layer
55
, an n-AlGaN cladding layer
56
, an n-GaN light guide layer
57
and an InGaN multiple quantum well (MQW) active layer
58
are successively provided on a sapphire substrate
51
.
Further, a p-AlGaN carrier blocking layer
59
, a p-GaN light guide layer
60
and a p-AlGaN first cladding layer
61
are successively provided on the active layer
58
. An n-GaN current blocking layer
62
having a striped opening
63
is formed on the p-AlGaN first cladding layer
61
. A p-AlGaN second cladding layer
64
and a p-GaN contact layer
65
are successively provided on the p-AlGaN first cladding layer
61
located in the striped opening
63
and the n-GaN current blocking layer
62
.
Partial regions of the layers from the p-GaN contact layer
65
to the n-GaN layer
54
are removed by etching, to expose the n-GaN layer
54
. A p type electrode
66
is formed on the upper surface of the p-GaN contact layer
65
, and an n type electrode
67
is formed on the exposed upper surface of the n-GaN layer
54
.
In the semiconductor laser device shown in
FIG. 12
, electrons (negative carriers) supplied from the n type electrode
67
are injected into the active layer
58
through the n-GaN layer
54
, the n-anti-cracking layer
55
, the n-AlGaN cladding layer
56
and the n-GaN light guide layer
57
. Holes (positive carriers) supplied from the p type electrode
66
are injected into the active layer
58
through the p-GaN contact layer
65
, the p-AlGaN second cladding layer
64
, the p-AlGaN first cladding layer
61
, the p-GaN light guide layer
60
and the p-AlGaN carrier blocking layer
59
.
The n-GaN current blocking layer
62
having the striped opening
63
is provided in order to reduce operating current and limit an emission spot position by limiting the flow of current in a striped manner. The n-GaN current blocking layer
62
limits the current flowing into the active layer
58
substantially to the region located under the striped opening
63
.
In the aforementioned conventional semiconductor laser device, however, the n-GaN current blocking layer
62
, having a larger refractive index as compared with the p-AlGaN first cladding layer
61
and the p-AlGaN second cladding layer
64
, has no effect of light confinement.
In order to bring the semiconductor laser device into a real refractive index guided structure for attaining the effect of light confinement, the n-current blocking layer must be made of n-AlGaN, for example, in a larger Al composition ratio than the p-AlGaN first cladding layer
61
and the p-AlGaN second cladding layer
64
so that the refractive index thereof is smaller than those of the p-AlGaN first cladding layer
61
and the p-AlGaN second cladding layer
64
. Thus, the effective refractive index in the region of the active layer
58
located under the n-current blocking layer is smaller than that of the region of the active layer
58
located under the p-AlGaN second cladding layer
64
in the striped opening
63
. Consequently, light is confined in the central portion of the active layer
58
. In this case, however, the p-AlGaN first cladding layer
61
and the p-AlGaN second cladding layer
64
, which are also made of n-AlGaN, and the n-AlGaN current blocking layer provide a large AlGaN film thickness in total. Such a film having a large Al composition is readily cracked if the thickness thereof is too large.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device having a current blocking layer, which is excellent in thermal stability and prevented from cracking.
Another object of the present invention is to provide a semiconductor laser device having a current blocking layer, which is excellent in thermal stability, prevented from cracking and improved in effect of light confinement.
A semiconductor laser device according to an aspect of the present invention comprises a first nitride based semiconductor layer including an active layer and containing at least one of boron, aluminum, gallium, indium and thallium, a current blocking layer, formed on the first nitride based semiconductor layer, having a striped opening, and a second nitride based semiconductor layer, formed on the first nitride based semiconductor layer in the striped opening, containing at least one of boron, aluminum, gallium, indium and thallium, and the current blocking layer includes a multilayer structure of at least one first layer of a nitride based semiconductor containing at least one of aluminum and boron and at least one second layer of a nitride based semiconductor containing indium and having a smaller band gap than the first layer.
In this semiconductor laser device, the first layer of the current blocking layer contains at least one of boron and aluminum, whereby the band gap of the first layer can be increased for reducing the refractive index of the first layer. Thus, in the case of a real refractive index guided structure, the difference in refractive index between the current blocking layer and the second nitride based semiconductor layer in the striped opening can be increased. Further, the first layer of the current blocking layer containing at least one of boron and aluminum is thermally stabilized. In addition, the second layer of the current blocking layer containing indium can absorb strain caused in the first layer containing boron or aluminum. Thus, cracking is suppressed.
Therefore, a semiconductor laser device having a real refractive index guided structure excellent in thermal stability, prevented from cracking and improved in effect of light confinement is implemented. Alternatively, a semiconductor laser device having a loss guided structure excellent in thermal stability and prevented from cracking is implemented.
At least one first layer may have a larger aluminum composition ratio than that of at least one second layer or a larger boron composition ratio than that of at least one second layer, and at least one second layer may have a larger indium composition ratio than that of at least one first layer.
In this case, at least one first layer is reduced in refractive index and improved in thermal stability. Further, at least one second layer absorbs strain of at least one first layer.
The first nitride based semiconductor layer may include a first conductivity type cladding layer, the active layer and a second conductivity type first cladding layer in this order, and the second nitride based semiconductor layer may include a second conductivity type second cladding layer.
At least one first layer may include at least two first layers, and at least two first layers and at least one second layer may be alternately stacked.
In this case, the second layer held between the first layers effectively absorbs strain caused in the first layers arranged on both sides thereof.
The mean refractive index of the current blocking layer may be small

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